Laminates of nylon and polyethylene are widely used for packaging various products and have been found especially suitable for foodstuff containing packages such as bags or pouches subjected to widely varying temperatures and conditions. Nylon is used for such packages because it is an oxygen barrier, has a high melting point, and is strong and clear. Polyethylene is used as an inner surface for such laminates because it is easily heat sealable, is moisture impermeable, and is relatively chemically inert to many food stuffs.
The disadvantages of nylon are principally its high cost, moisture permeability, and poor heat sealability; but, these disadvantages are largely offset by polyethylene's low cost, low moisture permeability, and good heat sealability. While the complimentary characteristics of nylon and polyethylene make them suitable for use as laminates, especially for packages containing food, the materials are somewhat incompatible because they are considerably difficult to initially bond together. This difficulty is thought to be attributable to their differences in physical and chemical structures. Also, once joined, the two materials will often separate upon physical deformation, particularly when agitated at water cooking temperatures which generally range from 140.degree. F upwardly.
A number of techniques have been developed in an attempt to overcome the bonding difficulties of nylon to polyethylene. These techniques include chemically and electrically treating the surface of the polyethylene and the incorporation of a layer of adhesive between the nylon and polyethylene. Some of the adhesives used in the prior art are ethylene-acrylic acid copolymers and the zinc or magnesium neutralized ionic copolymers known as ionomers. Even blends of an ionomer and polyethylene have been used in an attempt to find an adhesive that maintains sufficient cohesion in hot water or at boiling water temperatures. However, in an elevated temperature medium such as hot or boiling water, pouches, bags, and clipped casings of bulk packaged food, particularly packages containing more than ten pounds of food, all fail because the amide/olefin polymeric laminates tend to delaminate and shred within a short period of time. Accordingly, it is a principal object of the present invention to provide an amide/olefin polymeric laminate which will not delaminate under physical deformation at elevated temperatures.
It is another object of this invention to provide a laminated film in tubular or casing form which is suitable for packaging foodstuffs.
It is another object of the subject invention to provide a package in which food may be both cooked and stored without opening the package between the cooking and storing phases.
These and other objects are accomplished by the invention which is described herein below.